1. Field of the Invention
This invention relates to an EVA-based (ethylene vinyl acetate copolymer) foamable composition and a process for making a biodegradable EVA foam which can be used for the production of plastic articles, such as shoe soles, packing material, toys, medical equipment, etc.
2. Description of the Related Art
Polymers, such as polyethylene, ethylene vinyl acetate copolymer (EVA), ethylene acrylic acid copolymer, polypropylene, polystyrene, polyvinylchloride (PVC), etc., are non-biodegradable. Since large amounts of plastic wastes, which are made of these polymers, are produced each day, they have caused serious problems to the environment. These plastic wastes are very stable, and do not decompose for several decades after landfill disposal. Thus, there is a constant need for plastic materials that are biodegradable.
Starch is the most common among biodegradable components used in production of biodegradable plastics. U.S. Pat. No. 5,449,708 discloses a method for the continuous preparation of a starch-based biodegradable polymer material which contains an ethylene acrylic acid copolymer, a salt of stearic acid and an effective amount of starch. The biodegradable polymer material can be used in the formation of thin films, hollow tubing or rods, and the like. U.S. Pat. No. 4,133,784 discloses a flexible, self-supporting, and biodegradable film composition comprising a dry composite of a starchy material and a water-dispersible ethylene acrylic acid copolymer. The biodegradable film composition can be used in the formation of plastic films for packaging, such as for agricultural mulching.
However, such starch-based biodegradable polymer materials can only be used in the production of film type or tube type biodegradable plastics, and are not suitable for the production of plastic articles, such as shoe soles, packing material, toys, and medical equipment that require higher impact strength and hardness than those of the film-type or tube-type plastics.
U.S. Pat. No. 4,396,568 discloses a method for the manufacture of a relief-like profiled outer sole of a cross-linked ethylene-co-vinyl acetate (EVA) foamed with closed cells. However, the outer sole is not biodegradable.